In clinical application, computed tomography devices are used to acquire data records for an X-ray image which can be a specific material within an object to be examined or a patient. The aspect of material determination is becoming ever more important in routine clinical problems and so the importance and field of application of computed tomography devices has greatly increased.
A computed tomography device used to recognize material during the evaluation and display of X-ray data operates on the principle of the so-called dual energy method. In this method, the object to be examined or the patient is, for example, scanned with X-quanta up to 80 keV and with X-ray quanta up to 140 keV. As a result of the different X-ray spectra of the two types of X-ray radiation, a different average attenuation is generated such that more comprehensive information is gathered compared to a conventional computed tomography device in normal operation.
In this context, it is either possible for two X-ray radiators with different energy to be used during a scan or for the tube voltage of an X-ray radiator to be alternated between two scans or positions of the X-ray radiator executed directly one after the other. The different tube voltage of the two scans results a change to the X-ray spectrum required for the dual-energy method.
This change to the X-ray spectrum can also be intensified by an X-ray filter arranged in the beam path during one of the two scans or positions. A further alternative consists in the at least partial attenuation of the X-ray radiation with a given energy from an X-ray source by the selective use of an X-ray filter in the beam path of the X-ray source in order to generate X-ray radiation with variable energy. In addition to a software expansion, an alternative of this kind also requires the expansion of the hardware of the computed tomography system, wherein a separate moving mechanism is required for moving and positioning the X-ray filter. For example, US 2008/0198963 A1 and US 2005/0220265 A1 disclose X-ray systems for dual energy visualization in which a rotating X-ray filter can be positioned in the X-ray beam such that the spectrum of the X-ray radiation changes over time as a result of the rotation of the X-ray filter. Here, recordings are made alternately for each position, wherein each of the recordings corresponds to a different X-ray spectrum.
A slotted plate for limiting and shaping an X-ray fan beam is known from the U.S. Pat. No. 4,277,685 A, comprising several slotted openings with respectively different X-ray filtering regions for differentiating the X-ray spectrum of an incident X-ray beam, wherein the X-ray filtering regions in the area of the slotted openings are fixedly arranged.
A slotted plate is known from the DE 102 44 898 A1 for limiting and shaping an X-ray fan beam, said plate comprising several slotted openings with respectively different slot widths.